Andrew Chappell, Dibb Lupton Alsop's head of operations, is to quit the firm at the end of the year to pursue a different career. Chappell announced his decision at a management board meeting last week.
The Bradford-based partner, who ran the firm jointly with managing partner Nigel Knowles, told Legal Week: "Latterly I have not found the job fulfiling. We all need job satisfaction."
He said he had explored other options at the firm with Dibbs' management but said they would have been a "step backward". Chappell said that after he leaves in December he plans to develop an online stamp collecting service with his father.
He said a lot of people at the firm had been shocked by the news and he had had "a very positive reaction to what I contributed to the firm which is very gratifying". He added: "Dibbs has been tremendous to me."
Chappell is 45 and has been with the firm for 20 years. He became head of operations nearly two years ago in a power sharing arrangement with Knowles. The post was designed to foster a more collegiate culture in the firm.
In a statement, Dibbs said it was considering whether the formal position of operations partner would be maintained.
Knowles said: "The firm will always be grateful for the enormous contribution Andrew has made."